This invention relates to a method of metallizing integrated circuits with aluminum.
It is common for protrusions to form on the surface of thin metal film, particularly where a soft metal with a high coefficient of thermal expansion is deposited on a substrate with a significantly lower thermal expansion coefficient. Such a suituation exists in the case of aluminum integrated-circuit metallizations deposited on thermally-oxidized silicon, the thermal expansion coefficients of aluminum, silicon and SiO.sub.2 being 2.4.times.10.sup.-5, 2.5.times.10.sup.-6, and 5.0.times.10.sup.-7 .degree.C..sup.-1, respectively. Protrusions usually appear on the aluminum film surface after the high-temperature alloying process which establishes ohmic contact between aluminum and silicon in the circuit's contact cuts (also known as contact openings). The contact cuts as known in the art are developed by pattern etching the thermally oxidized silicon by photolithographic processes and the like. The combination of the high compressive stress induced by the mismatch in expression coefficients, and the low yield strength of aluminum at alloying temperatures (usually greater than 450.degree. C.) causes the film to yield, forming local protrusions at grain-boundary intersections which can reach heights greater than the film's thickness, typically one micrometer (.mu.m). The most common protruding form is the "hillock", a faceted, roughly pyramidal structure whose cross-section decreases with distance from its base. Another form, which looks like a short, fat "whisker", is also occasionally encountered.
Hillocks and whiskers are particularly troublesome in post-alloy integrated-circuit processing. The phosphosilicate-glass (PSG) overcoat typically deposited after metal alloying follows the contours of the hillocks. When photoresist is spun on in preparation for etching away the PSG from bond-pad areas, it often fails to cover the PSG at hillock sites. Consequently, the PSG is etched at such sites and aluminum is exposed, thereby creating a potential corrosion problem. Attempts to solve this problem have included: (1) application of a second photoresist coating before optical exposure of the first coating, or (2) application, exposure and development of a second coating after exposure, development and hard-backing of the first coating. These solutions have not been wholly satisfactory. A substantially protrusion-free metallization would elimate the need for such extra steps. A substantially protrusion-free surface for the purposes of this disclosure shall be understood to be that in which hillocks, etc., have a height less than 0.5 .mu.m. Applications may require a different criterion of what is protrusion free. Nevertheless, it should be understood that there is no purpose served in providing a perfectly smooth surface that is literally protrusion-free.
Aluminum hillock growth has been inhibited by the addition of alloying elements such as copper and silicon. However, copper increases the film's susceptibility to corrosion, and silicon adds to the complexity of film processing and properties. An alloy of aluminum and silicon (Al-Si) often requires an additional process step, i.e., "freckle etching" after metal pattern definition, to remove the silicon precipitates (so-called "freckles") from the SiO.sub.2 surface. These precipitates also represent a potential process-control problem, large variations in precipitate size and density having been observed in patterned metallizations on wafers which had experienced "identical" processing conditions. Large wafer-to-wafer variations in Al-Si etch rate for wafers within the same deposition run have been observed, and severe bondability problems have been periodically encountered.
Given these deficiencies, a system in which the added element is tightly bonded to the aluminum could provide an attractive alternative to aluminum, Al-Si and Al-Cu in many applications provided that (1) the additive element can be introduced in a controlled and reproducible manner, (2) the addition can be accomplished in existing deposition systems without major system modification or complication (3) the resulting film has low resistivity i.e. less than approximately twice the resistivity of bulk aluminum, and (4) the film is significantly less susceptible to hillock growth than aluminum films. Such applications would include (1) circuits requiring high (.gtoreq.500.degree. C.) alloying temperatures and (2) circuits which incorporate two separate levels of aluminum-based interconnects.
Aluminum films deposited under typical vacuum-system conditions (without deliberate addition of oxygen) contain on the order of one tenth of a percent oxygen. This oxygen, the source of which is the residual water vapor in the vacuum system, is probably helpful in reducing the stress in the as-deposited film and consequently in improving film adhesion to the integrated-circuit substrate. Since oxygen forms strong bonds with aluminum, it is also expected to be stable against the gross solute redistribution and precipitation which occur in Al-Si and Al-Cu alloy systems during post-desposition processes. Al-O films, i.e., films in which oxygen is deliberately added, have been shown to provide significant increases in resistance to electromigration in bipolar integrated circuits. See an article by H. J. Bhatt, entitled "Superior Aluminum For Interconnections Of Integrated Circuits", Applied Physics Letters 19, 30 (1971) which reports that such films containing Al.sub.2 O.sub.3 either did not form hillocks, or formed very few, smaller hillocks. Bhatt described depositing aluminum films with oxygen supplied during the entire time of deposition whereby a single layer of such a mixture is deposited.
Recently, a pulsed gas process has been developed to deposit high-strength, free-standing, thick (20-35 .mu.m), layered Al/Al.sub.x O.sub.y foils. This process is described by R. W. Springer and D. S. Catlett in a paper entitled "Structure and Mechanical Properties of Al/Al.sub.x O.sub.y Vacuum Deposited Laminates", Thin Solid Films 54 (1978), pp. 197-205. In particular, the Springer paper teaches the production of high strength foils through the lamination of aluminum and aluminum oxide layers, the entire foil having an overall thickness of 20 micrometers (200,000 angstroms) and thicker. The objective of the laminate structure described by Springer requires a high oxygen content for the Al.sub.x O.sub.y layers indicating that the experimentally achieved oxygen content in the order of 5 to 10% would have to be further increased to realize the full high-strength potential of the foil. Further discussions of this process of Springer et al. are presented in the J. Vacuum Sci. Technol., Vol 17, No. 1, January/February 1980, pp. 437-440 in an article entitled "Quantitative Characterization of High Strength Aluminum Foils Vapor Deposited on Curved Surfaces." A still further discussion of this process was included in a paper presented at the 9th Annual Symposium of the Applied Vacuum Science and Technology at Tampa, Florida on February 11-13, 1980. An abstract of the paper published on page 15 of the proceedings indicates that the laminate of thick aluminum foils were fabricated having thicknesses of 20 microns (200,000 angstroms).
A discussion of the internal stress in evaporated films is made in the Handbook of Thin Film Technology, Ed., Maissel and Glang, McGraw Hill, 1970, Chapter 12, page 36, with particular reference to Turner for his work using materials in alternating layers with compressive and tensile stress to increase the total thickness to which optical coatings could be deposited from 6 to 40 micrometers.